1. Field of the Invention
This invention relates to an apparatus for reading out an image stored in a stimulable phosphor sheet by two-dimensionally scanning the stimulable phosphor sheet by stimulating rays. This invention particularly relates to an apparatus for reading out an image by scanning wherein backward scan lines, i.e. return scan lines, in forward and backward scanning in the main scanning direction are erased.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
In an apparatus for reading out an image by scanning in the aforesaid radiation image recording and reproducing system, the stimulable phosphor sheet carrying a radiation image stored therein is two-dimensionally scanned by stimulating rays such as a laser beam, and light emitted by the stimulable phosphor sheet is sequentially detected and converted into electric image signals by a photodetector such as a photomultiplier. The two-dimensional scanning is conducted by scanning the stimulable phosphor sheet by stimulating rays in the main scanning direction and in the sub-scanning direction. For this purpose, in general, the stimulable phosphor sheet is moved in the sub-scanning direction and, at the same time, the stimulating rays are moved forwardly and backwardly in the main scanning direction normal to the moving direction of the sheet.
As a means for conducting the scanning in the main scanning direction, a galvanometer mirror moveable forwardly and backwardly at a predetermined speed along a predetermined path is generally used. In this case, since the stimulable phosphor sheet is continuously moved at a predetermined speed in the sub-scanning direction, the scan lines traced on the sheet by the stimulating rays moved forwardly and backwardly in the main scanning direction by the main scanning means zigzag at a predetermined angle with respect to the direction normal to the sub-scanning direction. Since the beam of stimulating rays has a predetermined diameter, the scan lines overlap at portions where the scanning direction is changed over from the forward scanning direction to the backward scanning direction and vice versa. Particularly when the scan line density is increased to conduct image read-out at a high accuracy, the forward scan lines and the backward scan lines become closer to each other and, therefore, the areas of the overlapping portions of the scan lines increase. When the stimulable phosphor sheet is once exposed to stimulating rays to emit light in proportion to the radiation energy stored therein, the level of the radiation energy stored decreases. Therefore, when overlapping of the scan lines arises as described above during image read-out, the problem that the portions of the stimulable phosphor sheet previously exposed to stimulating rays to release the stored radiation energy as light emission and now having a decreased level of residual radiation energy stored therein are again exposed to stimulating rays for image read-out arises in the overlapping portions of the scan lines. As a result, the electric image signals obtained by scanning the stimulable phosphor sheet by stimulating rays in this manner become incorrect.